Tongue movement depends on the actions of both intrinsic (origin and insertion in the tongue) and extrinsic (attached to bone and inserted into the tongue) muscles. These muscles play a key role in swallowing, breathing, chewing, and speaking. Contraction of the extrinsic muscles is generally considered to change tongue position (protrusion or retrusion), whereas contraction of the intrinsic tongue muscles changes tongue shape. To date, research that examines the respiratory-related effects of tongue function in mammals has focused exclusively on the respiratory control and function of the extrinsic tongue muscles. The respiratory-related control and function of the intrinsic tongue muscles and their bearing on extrinsic tongue muscle activity are still unknown. Recent findings indicate that the intrinsic tongue muscles may contribute to tongue protrusion and retraction, and facilitate the actions of the extrinsic tongue muscles in swallowing. In light of these findings, our objective is to characterize the respiratory-related activities of the intrinsic tongue muscles in vivo. The specific goals of the present application are to test the following hypotheses: (1) intrinsic tongue muscles are co-activated with extrinsic tongue muscles during resting tidal breathing; (2) intrinsic and extrinsic tongue muscle activities are modulated in parallel by central and peripheral chemoreceptors and airway mechanoreceptors; and (3) the EMG of intrinsic and extrinsic tongue muscles exhibit similar onset times and burst characteristics during perturbations of chemoreceptor and mechanoreceptor feedback. Experiments will be conducted on urethane anesthetized, spontaneously breathing male Sprague-Dawley rats. Simultaneous EMG recordings of the hyoglossus, internal intercostal muscles and superior longitudinal muscles will be obtained under each of the following conditions: (1) hypoxia, hypercapnia, and asphyxia, to assess the effects of central and peripheral chemoreceptor stimulation of intrinsic tongue muscle activities; (2) before and after superior laryngeal nerve section, and before and after lingual nerve section, to quantify the influence of upper airway mechanosensory modulation of intrinsic tongue muscle activities; (3) with and without single-breath airway occlusion, to quantify the influence of phasic lung volume changes on drive to intrinsic tongue musculature. The results of this work will enhance our understanding of the functions of the tongue musculature and provide broad insights into the modulation of tongue muscle activities in breathing and other behaviors such as chewing and swallowing.